1. Field of the Invention
The invention relates to an apparatus, specifically a circuit, that performs a three-wire ratiometric to two-wire excitation conversion.
2. Description of the Prior Art
Thermometry has many applications that cross a wide variety of technical disciplines including, but not limited to, temperature measurement, control and compensation. In one application, a thermistor type cardiac catheter is used to measure blood temperature for thermodilution studies. A saline or dextrose solution, having a known volume and temperature, is injected into the blood stream through one of the catheter lumens. The solution mixes with the blood and is diluted as it is carried downstream past a thermistor located at the surface of another catheter lumen. At the thermistor location, the temperature of the blood-injectate mixture is measured over a period of time. The cardiac output (efficiency) is computed from the temperature-time response data. Such temperature measurement depends, at least in part, on a resistance-temperature characteristic of the thermistor, also known as the beta characteristic.
The Association for the Advancement of Medical Instrumentation (AAMI) provides standards for the operation of electronic devices in medical equipment. These standards help to ensure the safety of the patient. For example, a catheter used to measure the temperature of blood inside a blood vessel will have more stringent operational parameters than a catheter used to monitor the body temperature rectally.
There are various types of catheters and connectors for the catheters. A first type of catheter that is used for cardiac thermodilution studies has a three-wire connector and a ratiometric response. This catheter operates under tight tolerances to ensure the safety of the patient. The current is kept to less than 50 microamperes and the voltage is kept to less than 800 millivolts. This catheter is very reliable and accurate, and is available at a low cost.
A second type of catheter that can be used for measuring body temperature, for example, rectally, has a two wire connector and a resistive response. The operational characteristics for the second type of catheter are not as stringent as for the first type of catheter. Therefore, this catheter cannot be used for cardiac thermodilution studies. Moreover, the second type of catheter may cost more than the first type of catheter.
In addition, the thermistors used in the first and second types of catheters may have different resistance-temperature characteristics. Over any specified temperature range for which the slope of a given material system curve may be considered to be constant, the resistance of a thermistor at any temperature within the specified range may be expressed as:             R      T        =                  R        TO            ⁢              xe2x80x83            ⁢              exp        ⁢                  xe2x80x83                [                              β            ⁡                          (                                                T                  o                                -                T                            )                                            T            ⁢                          xe2x80x83                        ⁢                          T              o                                      ]              ,
where RT is the resistance at an absolute temperature T expressed in kelvins (xc2x0 C.+273.15); xcex2 is the xe2x80x9cbetaxe2x80x9d or xe2x80x9cmaterial constantxe2x80x9d, and represents the slope of the thermistor""s resistance-temperature characteristic (in kelvins) over the specified temperature range; and, RTO is the resistance at a specified reference temperature, To that is also expressed in kelvins. Typically thermistor manufacturers provide beta information for their devices.
Monitors that attach to the second type of catheter provide a two-wire connector and expect a resistive response, and not a three wire connector with a ratiometric response. In addition, these monitors are designed for a specific resistance-temperature characteristic provided by a particular two-wire catheter. Therefore, catheters having a three-wire connector cannot be used with such monitors because of the differences in the connector and resistance-temperature characteristic. This increases cost and inconvenience because medical service providers need to have different types of catheters for the different monitors and measurements.
Therefore, there is a need for an apparatus that allows the three-wire catheter with the ratiometric response to be used with monitors having two-wire connectors and expecting a resistive response. This apparatus should also provide a two-wire response signal having a particular resistance-temperature characteristic over a predetermined temperature range.
The present invention satisfies this need by providing an impedance converter module that allows a 3-wire device with a ratiometric response to be used with a 2-wire monitor expecting a resistive response. An impedance converter module has a phase reversal circuit to receive a 2-wire excitation signal at first and second terminals, and to provide a polarized excitation signal with a predetermined polarity based on the 2-wire excitation signal. A series current path has a sense resistor connected in series with a variable impedance source. The polarized excitation signal is applied across the series current path. An output-voltage-sense circuit provides an output-voltage-sense signal based on a voltage across the series current path. A current mirror provides a drive signal to a ratiometric device based on current flowing through the series current path. Differential amplifiers receive a ratiometric response signal from the ratiometric device, and output measurement signals based on the ratiometric signal. A summing node combines the measurement signals to provide a single-ended ratiometric signal. The variable impedance source is controlled to provide a two-wire response signal at the 2-wire terminals based on the single-ended ratiometric signal and the output-voltage-sense signal.
In this way, the impedance converter module allows a device having a ratiometric response to be used with a monitor expecting a resistive response. The three-wire ratiometric signal from the device is converted to a two-wire output signal that has a desired output resistance over a predetermined range.